1. Field of the Invention
The present invention relates to a micro electro-mechanical system (MEMS) optical scanner.
2. Description of the Related Art
The MEMS optical scanner has been used in an image display apparatus that displays a picture by performing a scan with light. The MEMS optical scanner is capable of displaying an image by performing the scan with the light by allowing a mirror to reflect the light while rotary driving the mirror centering on each of two axial directions.
For example, in Japanese Laid-open Patent Publication No. 2012-163828, a MEMS optical scanner is disclosed in which a mirror is supported by two main arms at each of both ends thereof, or by four main arms in total. Each of the main arms is coupled to one end of a sub-arm, in which a piezoelectric film is installed, and furthermore, the other end of each of the sub-arms is fixed to a frame. The mirror, the main arms, and the sub-arms are obtained in an integrated state by processing an integral silicon substrate. In this MEMS optical scanner, the mirror is driven through the sub-arms and the main arms by expanding and contracting the piezoelectric film and by applying a drive signal to the piezoelectric film. According to this configuration, it is possible to rotate the mirror centering on each of an H-axis, or an axis along a horizontal direction, and a V-axis, or an axis along a vertical direction.
In this MEMS optical scanner, rotary driving centering on the H-axis of the mirror is often performed at a relatively high scanning speed. From a viewpoint of efficiency, this rotary driving is performed in a resonance mode by giving an external force to the mirror such that it coincides with a resonant frequency of the silicon substrate, which is a material of the mirror. On the other hand, compared to the rotary driving centering on the H-axis of the mirror, rotary driving centering on the V-axis of the mirror is often performed at a low scanning speed. This rotary driving is performed in a non-resonance mode by stopping coincidence with the resonant frequency of the silicon substrate and by giving the external force to the mirror such that predetermined linearity and efficiency are secured. In this rotary driving, ramp wave driving having a relatively large part contributing to image display becomes possible, whereby the efficiency is relatively good.
The MEMS optical scanner disclosed in Japanese Laid-open Patent Publication No. 2012-163828, however, is poor in shock resistance and is sometimes damaged when a rotary movement centering on the V-axis is performed, whereby it is not capable of stably performing a scan with the light. In addition, stray light is caused, whereby it is not capable of performing the image display with a high image quality.
On the other hand, there is a MEMS optical scanner in which a silicon substrate is fixed on a flexible printed circuit board and a mirror is rotated centering on a V-axis by rotating the flexible printed circuit board centering on the V-axis. The flexible printed circuit board to be rotary moved has a very low resonant frequency and has shock resistance better than that of the silicon substrate. Therefore, in this MEMS optical scanner, compared to the MEMS optical scanner disclosed in Japanese Laid-open Patent Publication No. 2012-163828, although the shock resistance is secured, it is necessary to perform rotary driving centering on the V-axis of the mirror by coinciding with the low resonant frequency of the flexible printed circuit board. In addition, this rotary driving is sign wave driving. Compared to the ramp wave driving, the sign wave driving has a large part difficult to contribute to the image display, whereby it is not efficient.